June 15, 1991 Solar Neutrons
CGRO was launched during the peak of the solar activity cycle and several solar flares were observed by all four instruments. Besides gamma rays, the COMPTEL instrument can also detect neutrons and measure their energy. Neutron interactions occur within the instrument when a solar neutron elastically scatters off a hydrogen nucleus in the liquid scintillator of an upper detector module. The scattered neutron then interacts in one of the lower modules, providing the trigger signal necessary for a double-scatter event. The energy of the scattered neutron is deduced from its time of flight from the upper to lower detector, so then the neutron data can be treated the same as the gamma-ray data to produce images and spectra. Neutron emission from the June 15 flare was seen for 1.5 hours after the impulsive phase. The image depicted here represents the neutron flux measured by COMPTEL over the duration of the flare. Along with the gamma-ray data, this provides evidence that the sun is accelerating protons and ions for several hours. This phenomenon was not observed before CGRO and represents a new understanding of solar flares.
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